Mack Trucks sales slowed in May, driven by a slight decrease in deliveries across North America.

Mack delivered 2,230 heavy-duty trucks worldwide in May, up only 1.2 percent from 2,203 one year earlier, according to a report released Thursday by the Sweden-based Volvo Group, Mack's parent company.

Solid gains in South America — deliveries jumped from 53 to 144 over the last year — helped offset a 2.6 percent decline in North America, where deliveries declined from 2,051 in May 2014 to 1,998 last month.

Despite the decrease in North America in May, John Walsh, Mack's vice president of marketing pointed to year-to-date deliveries, which indicate the heavy-duty truck manufacturer is still well ahead of last year's production pace.

So far this year, Mack has delivered 11,125 trucks worldwide, up 1,095 — or nearly 11 percent — from the same period in 2014. In North America, 10,226 trucks have been delivered this year, a 9 percent increase over last year.

"Returning customers and new customers alike are drawn to our full range of reliable, fuel-efficient models that boost productivity and profitability," Walsh said in a statement.

All Mack trucks built for the North American market are assembled at the company's Lower Macungie Township plant. The facility has approximately 1,950 employees.

As demand for Mack trucks has increased, the company has hired about 290 employees at the Lower Macungie plant since January 2014.

And demand has increased this year across the heavy-duty truck industry in North America. For example, Volvo — Mack's parent company — expects the total North American retail market for heavy-duty trucks to increase to about 310,000 vehicles in 2015. In 2014, the market increased by 14 percent to 270,300 vehicles, according to Volvo.